thanks for the support.
What I did today before coming back here was to recheck the complete joomfish installation, language activivation and translation. I couldn't find any troublemaker and then decided to clean the cache open a new browser session and then it worked.....
I'm using ff 184.108.40.206 and ie 7, default settings, to test and except from (known) minor layout differences, things work smoothly now in both browsers.
I think this one can be closed.